1. Field of the Invention
This invention relates to frequency response processing for a radiation image, particularly a radiation image processing method in a radiation image recording and reproducing system for recording a radiation image on a stimulable phosphor as an intermediate medium, obtaining image signals from the radiation image, and reproducing the radiation image as a visible image on a recording medium by use of the image signals, and an apparatus for carrying out the method. This invention also relates to an X-ray image processing method for processing the signals representing original image densities detected from an X-ray image, which has been recorded on an original photograph by the irradiation of X-rays to an object, at the time the X-ray image is to be copied, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then two-dimensionally scanned by stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected by a photodetector and converted to electric image signals, and the radiation image of the object is reproduced as a visible image by use of the image signals on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into electric signals to reproduce a visible image on a recording medium or a display device.
In the case where the aforesaid radiation image recording and reproducing system is used for diagnosis of the human body, the radiation dose to the human body can be decreased markedly as compared with the conventional X-ray image recording diagnosis system.
However, as the dose of radiation irradiated to the object at the time of the image recording is decreased, adverse effects of quantum noise of radiation or the like on the radiation image increase. As a result, graininess of the image deteriorates, and the reproduced visible image becomes rough.
In order to improve the graininess, the apparatus may be devised as described below. For example, a blur image may be stored on the stimulable phosphor sheet at the time of the image recording by making the stimulable phosphor sheet thicker or by making larger the grains of the stimulable phosphor used in the stimulable phosphor sheet. Alternatively, the image may be blurred at the time of the image read-out by increasing the beam diameter of stimulating rays used for the scanning, or the read-out image may be blurred by feeding the read-out analog image signals into an analog filter. Fine control is necessary in order to improve the graininess while deterioration of the other image quality factors such as sharpness are being minimized. However, with the aforesaid approaches to the improvement of the graininess, the kind of the stimulable phosphor sheet must be increased, and the degree of freedom of the control is limited even though the kind of the stimulable phosphor sheet is increased. Also, the degree of freedom of the control is very low though the mechanism becomes complicated, and the control is possible only in the direction of flow of the sequential image signals (the direction of main scanning). On the other hand, in order to improve the graininess by image processing, frequency response processing may be carried out by use of FFT (fast Fourier transform), or the image may be digitally blurred by calculating a mean value of the image signals around each scanning point. With the method using FFT, the degree of freedom of the control is very high. However, with this method, the processing speed is too low to process large numbers of the image signals, and a high cost is required to increase the processing speed. With the method wherein the image is digitally blurred by use of the mean value, fine control cannot be achieved and the image is generally blurred excessively even though the processing can be carried out quickly.